1. Field of the Invention
This invention relates to a method of producing a vibration-isolating bushing which is mainly used by being assembled into an automobile suspsension system or the like.
2. Description of Related Art
In an automobile suspesnsion system, a cylindrical vibration-isolating bushing is generally used at a coupling part between the vehicle body and the suspension, namely the coupling part between a lower arm or other arm for supporting wheels and a vehicle body side member such as a frame with a view to vibration damping and shock absorbing. The cylindrical vibration-isolating bushing can also be used for an engine mount supporting or bearing the engine in a vibration-proof fashion.
A conventional vibration-isolating bushing is, for example, known in the art as shown in FIG. 8, wherein the bushing 100 is made up of an inner cylinder 101 and an outer cylinder 102 both made of metal and disposed concentrically, and a rubber elastomer 103 interposed between the both. When the bushing 100 is used, a shaft component 104 is inserted in the inner cylinder 101, which is in turn fastened to supporting components 105 such as brackets pinching it from both sides thereby to fix the inner cylinder. On the other hand, the outer cylinder 102 is inserted in an attachment hole 107 of another supporting component 106 to be fixed to it.
Since the inner cylinder 101 of the vibration-isolating bushing 100 is pinched and held at both extremities thereof by the supporting components 105 in this way, if the area of the edge surface 108 of the inner cylinder 101 is small, the surface pressure of the edge surface subjected to an axial force by the fastening becomes higher, with the result that the supporting members 105 such as brackets press formed by a plate material subside and a problem of bolts releasing is caused.
In order to suppress the surface pressure of the aforementioned edge surface against the axial force by fastening to less than a definite value, an approach of thickening the entire wall thickness of the straight-formed inner cylinder 101 is adopted thereby enlarging the areas of the edge surfaces 108 in contact with the supporting components 105. However, where such a thick-walled inner cylinder 101 is employed, the area of each of the edge surfaces 108 is enlarged, yet the problem is encountered that its weight is significantly increased.
Another approach of enlarging the area of the edge surface is also known, for example, as shown in FIG. 9, wherein a profiled inner cylinder 101 whose both extremities 109 are thicker-walled than the intermediate part is employed. In this case, however, because of the profiled shape of the inner cylinder 101, such a preformed material that is worked by forging, etc. is employed and hence a cost increase is entailed. Furthermore, when the rubber elastomer 103 is molded by vulcanization, the free length of each of edge surfaces 110 of the rubber elastomer 103 cannot be sufficiently secured from the respect of mold releasing after molding, etc. and consequently, there is a problem of deterioration in durability. More specifically stated, where the extremities 109 of the inner cylinder 101 are preliminarily formed with an enlarged diameter, in vulcanization molding the rubber elastomer 103, a small-diameter portion 111 inboard of the extremities 109 assumes an undercut shape relative to the mold releasing in the axial direction, so that edge surfaces 110 of the rubber elastomer 103 cannot be set at two dots-dash line positions 112 considering the mold releasing after molding. That""s why the areas of the edge surfaces 110 of the rubber elastomer 103 are too small to secure a sufficient free length to elastic deformation.
JP Patent Publication 1993-200438 A1 discloses, as shown in FIG. 10, upsetting the extremities 109 of the inner cylinder 101 by cold plastic working by the use of an upsetting jig 120 after vulcanization molding of the rubber elastomer 103 thereby enlarging the edge surfaces 108. In this reference, an upsetting jig 120 having at its top central surface a projection 121 is used and the edge surface 108 is enlarged by pressing the upsetting jig 120 onto the edge surface 108 of the inner cylinder while causing it to gyrate on a conical orbit centering the axis 122 of the inner cylinder 101 thereby enlarging the edge surface 108 as shown in FIG. 11. According to this method it is possible to enlarge the area of each edge surface 108 of the inner cylinder 101 without using any thick-walled tube and besides it is also possible to enlarge the edge surfaces 110 of the rubber elastomer 103 thereby to secure sufficiently a free length to the elastic deformation.
Nevertheless according to the upsetting method as disclosed in the reference, the extremity 109 of the inner cylinder 101 is enlarged in inside diameter d0 as well as outside diameter D0 as shown in FIG. 11, so that the area of the edge surface 108 of the inner cylinder is narrowed by the increment in inside diameter d0 and hence it is difficult to secure an area enough to reduce the surface area against the axial force.
In view of the prior art problems and difficulties above, this invention has been made and accordingly, a principal object of this invention is to provide a method of producing a vibration-isolating bushing with which it is possible to secure a much larger areas for each edge surface of the inner cylinder in upsetting the extremities of the inner cylinder by cold plastic working after vulcanization molding of the rubber elastomer than conventional method.
This invention provides a method of producing a vibration-isolating bushing comprising securing a rubber elastomer to an outer circumference of an inner cylinder by vulcanization molding and is characterized in that in upsetting axial extremities of the inner cylinder by cold plastic working after vulcanization molding, an upsetting jig whose top surface is substantially flat is used to as an upsetting jig for pressing onto each edge surface of the inner cylinder and the top surface of the upsetting jig is pressed onto the edge surface of the inner cylinder while causing it to gyrate on a conical orbit centering the axis of the inner cylinder, thereby enlarging the edge surface of the inner cylinder.
By conducting the upsetting of extremities of the inner cylinder by the use of such an upsetting jig that has at its top surface no projection, but is flat by reason of the pressing and gyration motion it is possible to upset the outside diameter side only while suppressing the upsetting at the inside diameter side and consequently, to secure sufficiently the areas of edge surfaces of the inner cylinder.
In the production method of this invention, alternatively, such an upsetting jig that its top surface is slightly bulged to assume a conical surface may be used and the upsetting jig can be tilted relative to the axis of the inner cylinder so that the conical surface may be nearly in parallel with the edge surfaces of the inner cylinder. This method is effective in case where only the outside diameter is upset and the inside diameter is hardly changed.
Further in accordance with the production method of the invention, a constraint wall for surrounding the outer circumference of each of the extremities of the inner cylinder to be upset may be vertically arranged at the peripheral portion of the top surface of the upsetting jig, thereby inhibiting the extremities from being enlarged beyond a predetermined diameter. This method is effective in case where the outside diameter is upset while the inside diameter is reduced, whereby it is possible to secure all the more the areas of edge surfaces of the inner cylinder.